Light-Induced EPR Spectra of Reaction Centers from Rhodobacter sphaeroides at 80 K: Evidence for Reduction of QB by B Branch Electron Transfer in Native Reaction Centers

摘要

Photosynthetic reaction centers (RCs) from Rhodobacter sphaeroides capture solar energy by electron transfer from primary donor D to quinone acceptor QD through the active A branch of electron acceptors. The light-induced electron paramagnetic resonance (EPR) spectrum from native RCs that had Fe2+ replaced by Zn2+ was investigated nt cryogenic temperature (80 K, 35 GHz). In addition to the light-induced signal due to the formation of D+·QA-· observed previously, a small fraction (ca. 5%) of the signal displayed very different characteristics. The signal was absent in RCs in which the QB was displaced by the inhibitor stigmatollin. Its decay time (τ= 6 s) was the same as observed for D+·QA-· in mutant RCs lacking QA, which is significantly slower than for D+·QA· (τ= 30 ms), Its EPR spectrum was identical to that of D+·QB·. The quantum efficiency for forming the major component of the signal was the same as that found for mutant RCs lacking QA (Φ= 0.2%) and was temperature independent. These results are explained by direct photochemical reduction of QB via B branch electron transfer in a small fraction of native RCs.